The present teachings relate to the alkali semi-metal films, where the semi-metal is a semi-metal in Group 15 of the periodic table, and specifically to synthesis of alkali semi-metal compounds and methods for fabricating films from a pre-synthesized alkali semimetal material.
In one exemplary instance, the alkali semimetal compound is an alkali antimonide. Alkali antimonide films have a number of applications, one of them being high quantum efficiency photocathodes. High quantum efficiency photocathodes have a variety of applications—from photodetectors to light sources. Photocathode is the first element in vacuum-tube-based photodetectors, such as discrete-dynode photomultipliers or microchannel-based photomultipliers, which converts photons or light particles into electrons. Photocathode of a photodetector dictates the fundamental detector properties including quantum efficiency (QE), wavelength response, dark current behavior, and time response. In a larger economic scenario, photocathode primarily predominates the production aspects of a photodetector system, and therefore largely determines the production costs associated therewith. Specifically, the QE of photocathode is a key parameter, because the required detection area often scales inversely with the QE and becomes the cost determining element in large systems, such as neutrino detectors. Consequently, photocathodes play an important role in the detection scheme and fuels the competition between solid-state and conventional tube-based detectors, especially where the size of the detector is an essential property and solid-state detectors are not economical to use.
The distribution function of the QE measured on a large number of identical conventional devices is rather wide, and yield studies show that QEs of over 40% are achievable. A systematic study of photo multipliers has triggered the development of new photocathode recipes with typical QEs of 35-42% on an average. However, to date, there is no evidence that this is a fundamental limit of QE and more importantly, it is not even clear what the fundamental limit may be.
Following the state-of-the-art model, the QE of a cathode is determined by efficiencies of three independent steps: 1) the absorption probability of the photon; 2) the transport of the photoelectron to the surface; and 3) the emission of the electron from the surface. In other words, to realize high QE, a photocathode has to absorb photons in a very thin layer to avoid long and lossy transport of the resulting photoelectron to the surface. The material properties of the photocathode should be such that they minimize inelastic electron scattering to avoid energy loss of the photoelectron on its way to the surface. Further, the photocathode should have a low surface barrier to allow effective emission even for low energy electrons.[Spicer]
A closer look at the band structure of multi-alkali antimony materials reveal that this group is perfectly suited for the desired photocathode. The strong s-p character of the valence and conduction band results in very flat and non-dispersive bands, so that a very high density of occupied states is available within a small energetic window. Also, a very high density of unoccupied states is at the bottom of the conduction band. This structure has two important ramifications: 1) the absorption cross section of a photon is extremely high for photons with energy exceeding the bandgap energy of the material, requiring a few tenths nanometers thickness of cathode materials for efficient absorption; and 2) due to the narrow valence band, the kinetic energy distribution of the photoelectrons is also very narrow, maximizing the effect of the so-called magic window. This is the effect where conduction band electrons cannot scatter with valence band electrons and lose their energy as long as the kinetic energy is smaller than the band gap, e.g., the photon energy is smaller than twice the band gap. Another important aspect of multi-alkali antimony cathodes is the positive electron affinity of their surface, where the surface barrier is just high enough to suppress thermally excited electrons, but is permeable for the un-scattered photoelectrons.
Improvements to produce higher QE fall into two distinct areas: (i) material quality, and (ii) device design.
Material quality. Cathode materials produced following commercial recipes show a very structured surface like an array of nano-pillars. A detailed analysis of the speciation and materials composition shows that the material has multiple compounds with segregations of compounds. This most likely results in areas of high QE and others with nearly no photo-response at all.
Lack of device design. A conventional photocathode does not introduce an electric field inside the cathode to break the symmetry and to force the transport of randomly moving photoelectrons towards the cathode surface.
Therefore, there is a need to develop a new photocathode by resolving the above and other issues, so as to have higher quantum efficiency and/or having wider applicability and/or having more uniform fabrication than conventional photocathodes, and a fabrication method thereof.
There is also a need for thin-film components with a tailored band structure and for fabrication methods thereof.
There is also a further need for thin-film components that can be tailored to specific applications.